A series of opioid ligands utilizing the di,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) fluorophores 4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-propionic acid or 4,4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza-s-indacene-3-propionic acid were synthesized and characterized for their ability to act as a suitable fluorescent label for the mu opioid receptor. All compounds displaced the mu opioid receptor binding of [H-3]Tyr-D-Ala-Gly-(Me)Phe Gly-ol in monkey brain membranes with high affinity. The binding of fluorescent ligands to delta and kappa receptors was highly variable. 5,7-Dimethyl-BODIPY naltrexamine, ''6-BNX,'' displayed subnanomolar affinities for the mu and kappa opioid receptors (K-i 0.07 and 0.43 nM, respectively) and nanomolar affinity at the delta (K-i 1.4 nM) receptor. Using fluorescence spectroscopy, the binding of 6-BNX in membranes from C-6 glioma cells transfected with the cloned mu opioid receptor was investigated. In these membranes containing a high receptor density (10-80 pmol/mg protein), 6-BNX labeling was saturable, mu opioid specific, stereoselective (as determined with the isomers dextrorphan and levorphanol), and more than 90% specific. The results describe a series of newly developed fluorescent ligands for the mu opioid receptor and the use of one of these ligands as a label for the cloned mu receptor. These ligands provide a new approach for studying the structural and biophysical nature of opioid receptors. (C) 1997 Elsevier Science Inc.
Selective and Wash‐Resistant Fluorescent Dihydrocodeinone Derivatives Allow Single‐Molecule Imaging of μ‐Opioid Receptor Dimerization
specific effects in living cells. We developed new fluorescent ligands based on the μ-OR antagonist E-p-nitrocinnamoylamino-dihydrocodeinone (CACO), that display high affinity, long residence time and pronounced selectivity. Using these ligands, we achieved single-molecule imaging of μ-ORs on the surface of living cells at physiological expression levels. Our results reveal a high heterogeneity in the